This book was a great reference, gathering together in one volume information that could be used to craft the religous structure for many different campaigns, by including fictional, mythological, and "real" gods and heroes in one book. Also included in the volume was then-new information about the planes of existence, planar travel, and other-planar creatures.
Those who complain about this book probably haven't read it. If they had, they would have seen in the intro that the authors did not intend this to be a book of adversaries (although they wrote up the avatar's stats for them if needed), but instead to be a quick reference for these different pantheons, some of which cannot be found in the reference section of the library (when did you last see a scholarly work on Lolth, Blibdoolpoolp, or Vaprak the Destroyer?). They also mention that if a particular mythos captures your interest, you should journey to your local library to research it. In other words, this is a game supplement, not a religious text.
Personally, I used this book a lot when I was running games back in the glory days of AD&D. I loved creating settings using the Norse or Celtic mythoi, and having the quick reference along with information about the avatars of the gods (important when dealing with gods with a "hands-on" approach such as the Norse) was at times quite useful. Also, it gave a starting point for research that would have otherwise taken longer, giving the names and general spheres of influence of a number of gods and goddesses that I was not then familiar with.
Yes, the use of still-used mythoi could be seen in this PC age as incorrect, but the only difference between a religion and a myth cycle is whether or not people still believe in it. To the Norse, their religion was all-encompassing, yet nobody complains these days about the comic book character The Mighty Thor. And look at the abuse the Greek mythos has taken in popular media, yet the Greeks manage to take it all in stride. Lighten up, people.
The description of the product doesn't say, so I am assuming what they are selling here is a second-edition or later book. The difference is that the first edition contained the Cthulhu and Melnibonean mythoi, based on the works of Lovecraft and Moorcock, respectively. The problem was, Chaosium (a rival games maker) already had the rights to those works, and TSR violated those rights when they printed their book. The material in question was removed for later printings, making the first edition an instant collector's item.